Optical recirculation depolarizer and method of depolarizing light

ABSTRACT

An optical depolarizer and method of depolarizing light is described. An input light beam is split into two beams. One split beam is recirculated through a birefringent medium and looped back to be recombined with the input light. This allows a weighted averaging of the different polarization states that result from birefringence in the recirculation path of the recirculated beam. The depolarizer is formed from single mode fiber optic cables and fused single mode fiber couplers. Each fiber coupler has an input pair of fibers and an output pair of fibers. One of the output fibers is coupled to one of the input fibers to form a recirculation loop. Additionally, polarization controllers provided in the input fiber and recirculation loop allow the degree of polarization of the output beam to be varied across a wide spectrum of values.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of optics in general.More particularly, the present invention relates to the field ofdepolarizers which have applications in communications, sensors, opticalinstruments and other areas.

[0003] 2. Description of the Related Art

[0004] Many optical devices used in communications and instrumentation,such as switches, couplers and modulators, are highly sensitive to thestate of polarization of light. The performance of communication systemsand instruments which utilize such optical devices varies as the stateof polarization (hereinafter SOP) varies. Fluctuations in the SOP canresult in reduced signal to noise ratios in fiber optic communicationsystems or decreased sensitivity and accuracy in fiber opticinstruments.

[0005] When light passes through a fiber optic cable (hereinafterfiber), the initial polarization of the light, whether polarizedelliptically or linearly, can be changed due to varying environmentalfactors affecting the fiber. These environmental factors produce changesin the index of refraction of the fiber. Light propagating along thefiber will pass through these regions having differing indexes ofrefraction, thereby changing the initial SOP of the light as itpropagates along the fiber. This effect of altering the SOP of light asit passes through a medium is called birefringence. The polarizationreceived at the output end of the fiber may thus change radically fromthe initial SOP at the input end of the fiber. Because birefringence isaffected by varying environmental factors, the output SOP will not havea predictable relation to the input SOP. Instruments and devicessensitive to SOP will therefore have their performance degraded in amanner which cannot easily be predicted or corrected.

[0006] One solution to the problem of birefringence is to replace commonsingle mode fiber with polarization maintaining fiber (hereinafter PMF),which is not sensitive to environmental factors and therefore preservesthe initial SOP as light propagates along the fiber. While PMF hasadvantages over standard single mode fiber, it is also very expensive touse. One meter of PMF costs approximately $10.00, roughly 100 times thecost of single mode fiber.

[0007] While single mode fiber has the drawback of birefringence, if theincident light propagating within the fiber is depolarized, then thebirefringent effect will not alter the SOP. Depolarized light is thecombination of light of all polarization states in equal proportion.Birefringence in single mode fiber alters all of the polarization statesequally, thereby preserving the depolarization of light propagatingalong the fiber. Thus, if depolarized light is used, birefringence nolonger produces a degradation in system performance.

[0008] One problem associated with utilizing depolarized light is thatlight sources used in fiber optic systems have a high degree ofpolarization (hereinafter DOP). The DOP is defined as the fraction ofoptical power that is polarized. To utilize polarized light sources, adepolarizer must be employed to remove the DOP. Currently availabledepolarizers have significant limitations which reduce their practicalapplicability in both fiber optic communication systems and fiber opticinstrumentation.

[0009] One type of depolarizer is the electro-optic pseudo-depolarizer,which utilizes electrodes positioned on either side of a waveguide tochange the refractive index within the waveguide. The varying refractiveindex in turn varies the SOP of the light passing through the waveguide.Although varying the refractive index of the waveguide varies the SOP,the measured effective DOP depends on detector speed. Over severalcycles of varying the refractive index of the waveguide, thetime-averaged output light appears depolarized in that no one SOP ispreferred during the averaging time. This form of depolarization iscalled pseudo-depolarization or time-averaged depolarization, and hasthe disadvantage that light exiting the depolarizer within a narrow timeinterval has a high DOP. High speed detectors, however, detect light ina narrow time interval. Thus, a high speed detector would capture lightwith a high DOP when the light is time averaged over the narrow timeinterval. Additionally, the electro-optic pseudo-depolarizer is anactive system requiring both driving circuitry and a power supply.Failure of any of these active components would result in the lightexiting the waveguide with a high DOP. Another drawback of theelectro-optic pseudo-depolarizer is its high cost. An electro-opticpseudo-depolarizer costs approximately $1000.00.

[0010] Another type of currently available depolarizer is the acousticdepolarizer. A driving speaker vibrates a segment of fiber within thedepolarizer, thereby altering the index of refraction within the fiberas the fiber bends and vibrates. The index of refraction within thefiber varies at the frequency of the speaker. Polarized light passingthrough the vibrating fiber has its SOP altered at the frequency of thespeaker. As with the electro-optic pseudo-depolarizer, the acousticdepolarizer depolarizes light on a time averaged basis. The DOP thenvaries at the frequency of the driving speaker. For detectors andinstruments which detect polarization states at a time interval narrowerthan the time interval for depolarization, which is dependent on thefrequency of the speaker, light exiting the acoustic depolarizer willhave a noticeable DOP. Another disadvantage of the driving speakerdepolarizer is that it is an active system which relies on theperformance of the driving speaker. In addition to the costs associatedwith the driving speaker and associated circuitry, such a system isprone to failure if any of the many components of the speaker or drivingcircuitry fail. Thus, although the driving speaker depolarizer reducesthe DOP, the output light still retains a significant DOP within anarrow time interval.

[0011] Another known depolarizer is the Lyot depolarizer. The Lyotdepolarizer consists of two plates of quartz crystal having largeretardances. The light source utilized with a Lyot depolarizer is abroad band source, for example a superluminescent diode. The crystalsare arranged such that the incident light passes through the firstcrystal and into a second crystal adjacent to the first crystal. Theratio of thickness of the two crystals is 2:1. While the light exitingthe second crystal is depolarized over a large wavelength region, lightin a small wavelength region is not depolarized. Thus, the Lyotdepolarizer is ineffective for depolarizing monochromatic or narrowwavelength light sources. Another drawback of the Lyot depolarizer isthe high cost associated with using a broad band light source. Broadband light sources have the additional disadvantage of having loweroutput power than is possible with narrow band light sources. Anotherdisadvantage of the Lyot depolarizer is its inapplicability with manyfiber optic communication systems due to the use of a broad band lightsource. As a pulse of light from a broad band source propagates alongthe birefringent single mode fiber used in many communications systems,there is a time shift in the pulse caused by the different propagationspeeds of the different wavelength components of the broad band lightpulse. This time shift causes a “spreading” of the light pulse and isincompatible with the high data transmission rates of many fiber opticcommunication systems.

[0012] Still another type of currently available depolarizer isdescribed in U.S. Pat. No. 5,486,916, issued to Michal et al., and inU.S. Pat. No. 5,457,756, issued to Hartl et al. This type of depolarizeris constructed from PMF. The ends of the PMF are oriented such thattheir principal axes are at an angle of 45°. In such a depolarizer thequality of the depolarizer depends critically on the 45° alignment ofthe PMF. As with the Lyot depolarizer, this type of depolarizer alsorequires a broad band light source. Thus, in addition to the high costof the depolarizer dictated by the use of a broad band light source andthe use of PMF, there are high fabrication costs associated withcritically aligning and fusing the PMF. As with the time averagingpseudo depolarizers described above, this depolarizer spectrum averagesand has the disadvantage in that it cannot be connected in series withother depolarizers of the same type. If it is connected in series withitself, as when two depolarizers are arranged such that the output ofone depolarizer is input a second depolarizer, the output light from thesecond depolarizer has its DOP increased from the output of the firstdepolarizer. Thus, the depolarizer described in U.S. Pat. No. 5,486,916is not suitable for producing very low DOP through series combination ofthe depolarizer.

[0013] Accordingly, it is desired that the present invention overcomethe limitations of current optical depolarizers.

SUMMARY OF THE INVENTION

[0014] The present invention provides an optical depolarizer and methodof depolarizing light, wherein light is depolarized by splitting thebeam into an output beam and a recirculation beam. The recirculationbeam propagates along a birefringent path and is then recombined withthe input beam before the input beam is split into the output beam andrecirculation beam. The combined input beam and recirculation beam isthen split into an output beam and a recirculation beam. This process ofsplitting, recirculating along a birefringent path and recombining withthe input beam averages beams with different states of polarization suchthat the output beam is the average of many light beams with differingstates of polarization.

[0015] In one embodiment of the present invention a birefringent elementis used with a plurality of mirrors. The input light beam is split intotwo beams by a partially reflecting mirror. One beam forms the outputbeam of the depolarizer. The other beam is reflected by mirrors througha birefringent element and back to the partially reflecting mirror. Thispart of the beam passing through the birefringent element forms therecirculation loop. Part of this beam striking the partially reflectingmirror is passed through as part of the output beam of the depolarizer.The other part is reflected along the path of the recirculation loop.

[0016] In one embodiment of the present invention the recirculationsegment is used where the recirculating beam is reflected back along thepath of the input beam and through a birefringent element along theinput beam. The recirculating beam is then reflected back along the pathof the input beam in the direction of the input beam.

[0017] In one embodiment of the present invention a 2×2 fiber coupler isused where one input fiber and one output fiber are connected to form arecirculation loop.

[0018] In one embodiment of the present invention polarizationcontrollers are included within the input fiber and recirculation loopto allow the degree of polarization to be tuned within a wide range ofvalues.

[0019] In another embodiment of the present invention multiplesingle-ring depolarizers of either the standard type or the tunable typeare connected is series such that the light output one single-ringdepolarizer is input the next single-ring depolarizer.

[0020] In another embodiment of the present invention multiple 2×2 fibercouplers are connected in a non-series arrangement to allowrecirculation and recombination of the light, thereby averagingdifferent polarization states and depolarizing the output light beam.

[0021] In another embodiment of the present invention a recirculationdepolarizer is formed from a single fiber coupled with itself to form arecirculation loop.

[0022] In another embodiment of the present invention a recirculationdepolarizer is formed as an integrated optical device on a substrate.Waveguides direct the input light beam along the recirculating path todepolarize the input light beam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a diagram of a free space recirculating depolarizer witha recirculation loop in accordance with the present invention;

[0024]FIG. 2 is a diagram of a free space recirculating depolarizer witha recirculation segment in accordance with the present invention;

[0025]FIG. 3 is a diagram of a single-ring fiber optic recirculatingdepolarizer in accordance with the present invention;

[0026]FIG. 4 is diagram of three single-ring fiber optic recirculatingdepolarizers as shown in FIG. 3 connected in series in accordance withthe present invention;

[0027]FIG. 5 is a diagram of a single-ring tunable fiber opticrecirculating depolarizer in accordance with the present invention;

[0028]FIG. 6 is a diagram of three tunable single-ring fiber opticrecirculating depolarizers as shown in FIG. 5 connected in series inaccordance with the present invention;

[0029]FIG. 7 is a diagram of three single-ring fiber optic recirculatingdepolarizers as shown in FIG. 3 connected in series with two tunablesingle-ring fiber optic recirculating depolarizers as shown in FIG. 5 inaccordance with the present invention;

[0030]FIG. 8 is a diagram of a multiple splitter recombination fiberoptic depolarizer in accordance with the present invention;

[0031]FIG. 9 shows a single ring depolarizer formed from a one piece offiber looped to form a recirculation loop in accordance with the presentinvention; and

[0032]FIG. 10 shows a recirculating depolarizer formed as an integratedoptical device in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0033] The present invention provides a method and apparatus fordepolarizing light. In the following description, numerous details areset forth in order to enable a thorough understanding of the presentinvention. However, it will be understood by those of ordinary skill inthe art that these specific details are not required in order topractice the invention. Well-known elements, devices, process steps andthe like are not set forth in detail in order to avoid obscuring thepresent invention. Further, all patents, technical papers and otherreferences referred to herein are incorporated by reference herein.

[0034]FIG. 1 is a diagram showing a free space depolarizer (2). Lightfrom a light source, not shown, is input to a partially reflectingmirror (4). One part of the input beam is reflected from the partiallyreflecting mirror (4) and forms the output beam (6). The arrows alongthe path of the beam indicate the direction of propagation. The otherpart of the input beam passes through the partially reflecting mirror(4) to a mirror (8). The beam passing through the partially reflectingmirror (4) to the mirror (8) is referred to as the recirculation beam.The mirror (8) reflects the recirculation beam to a mirror (10). Therecirculation beam reflected off the mirror (10) then passes through abirefringent element (12) and is reflected off a mirror (14). Thebirefringent element (12) can be formed from any translucent materialwhich alters the SOP of a beam of light passing though it. Afterreflecting off the mirror (14), the recirculation beam returns to thepartially reflecting mirror (4). As before, the light beam interceptingthe partially reflecting mirrors is split into two beams. One beampasses through the partially reflecting mirror (4) and becomes part ofthe output beam (6) of the depolarizer (2). The other part of therecirculating beam is reflected by the partially reflecting mirror (4)to become part of recirculation beam.

[0035] In this manner, light input to the depolarizer (2) has a portionof the beam passing through the birefringent element. The output beam ofthe depolarizer (2) is then a combination of the input beam and therecirculation beams. The recirculation beam is continually split by thepartially reflecting mirror (4), such that part of the recirculationcontinually beam passes through the birefringent element. Each timelight passes through the birefringent element the SOP of the beamchanges. Thus, the output beam, as the combination of many beams ofdiffering SOP, no longer has a high DOP associated with the SOP of theinput light. From the addition of many recirculation beams withdifferent SOP, the resulting output beam has a low DOP.

[0036] The partially reflective mirror (4) can be chosen to allow anyportion of the energy of the input beam to propagate along as therecirculation beam. Possible successful embodiments include a 50/50split where the input beam is divided into two beams of equal intensity,as well as a 67/33 split, where the first number represents thepercentage of light entering the recirculation beam and the secondnumber represents the percentage of light entering the output beam. Alow DOP has been shown with a 67/33 split, where 67% of the lightreaching the beam splitter is propagated along the recirculation loop.

[0037] While the embodiment shown in FIG. 1 utilized four mirrors andone birefringent element, other embodiments could have severalbirefringent elements and a different arrangement of mirrors. Anyarrangement of mirrors and birefringent elements that allows therecirculation and recombination of part of the input beam through abirefringent media would allow depolarization of the input light inaccordance with the present invention.

[0038]FIG. 2 shows a diagram of a free space depolarizer (16) whereinthe recirculation path is a linear segment. Light from a light source,not shown, is input to a two-way mirror (18), which allows the inputbeam to pass through in the direction of propagation of the input beam.After passing through the two-way mirror (18), the input beam passesthrough a birefringent element (20) which alters the SOP of the inputbeam. After passing through the birefringent element (20), the inputbeam strikes a partially reflective mirror (22). The partiallyreflective mirror (22) splits the input beam into two beams. One part ofthe input beam passes through the partially reflective mirror (22) andbecomes the output beam of the depolarizer (16). The other part of theinput beam is reflected back from the partially reflective mirror (22)as the recirculation beam. The recirculation beam then backtraces thepath of the input beam, first passing through the birefringent element(20), and then striking the two-way mirror (18). By backtracing it ismeant that the recirculation beam reflected from the partiallyreflecting mirror (22) travels back in the direction of the two-waymirror (18). The recirculation beam need not retrace the exact path ofthe beam. After striking the two-way mirror (18), the recirculation beamis then reflected back from the two-way mirror (18), through thebirefringent element (20), to strike the partially reflective mirror(22). As before, the recirculation beam is continually reflected betweenthe mirrors. In the depolarizer (16) the path between the two-way mirror(18) and the partially reflective mirror (22), including thebirefringent element (20), is the recirculation path of the depolarizer.Because the recirculation path of this depolarizer follows the path ofthe input beam, rather than a separate recirculation loop as in thedepolarizer (2) shown in FIG. 1, the recirculation path is referred toas a recirculation segment.

[0039] A polarized input light beam entering the depolarizer (16) willhave its DOP reduced in the following manner. As the input beam passesthrough the depolarizer (16) along the recirculation segment, the SOP ofthe input beam will be altered due to the birefringence of therecirculation segment, but the DOP will remain the same. Therecirculation beams which traverse back along the recirculation segmentwill have their SOP varied from the SOP of the input beam due to thebirefringence of the recirculation segment. After the recirculation beamis reflected back from the two-way mirror (18), the input beampropagating through the two-way mirror, and the recirculation beamreflected from the two-way mirror, combine to form a beam of decreasedDOP, as the combination of two beams with different SOP. When thecombined beam strikes the partially reflecting mirror (22), thereflected beam returns along the recirculation segment and is reflectedby the two-way mirror (18). The combined beam has a lower DOP than theinput beam. The part of the combined beam passing through the partiallyreflective mirror (22) has a lower DOP with each successive addition ofa recirculation beam.

[0040] Although the above embodiments show a depolarizer with abirefringent element separate from the mirrors of the depolarizer, otherembodiments could have the birefringent element as part of the mirror ormirrors of the depolarizer.

[0041] While the embodiment shown in FIG. 2 uses separate mirrors and aseparate birefringent element, one way of constructing such adepolarizer includes using a single fiber which has been grated. Asection of the fiber is grated to produce a first grating which allowsthe input beam traveling along the fiber to pass through. Further alongthe fiber in the direction of propagation of the input beam a secondgrated section is formed. This second grated section reflects a portionof the beam striking it, while allowing a portion of the beam to passthrough. The segment of fiber between the first grating section and thesecond grating section acts as a birefringent element to alter the SOPof light which propagates along it. The first grated section, the secondgrated section and the segment of fiber between the grated sections formthe recirculation segment. The input beam of the fiber passes throughthe first grated section to the second grated section. The input beamreflected back from the second grated section along the fiber to thefirst grated section, where the light is reflected again. When the lightis reflected back from the first grated section it is combined with theinput beam traveling along the fiber. A portion of this combined beampasses through the second grated section and forms the output beam ofthe depolarizer. As before, a portion of the combined beam is reflectedback from the second grated section to the first grated section, therebyrepeating the process of splitting and recombining with the input beam.The combining of beams with different SOP reduces the DOP of the outputbeam.

[0042]FIG. 3 is a diagram showing a single-ring recirculatingdepolarizer (24) constructed from a standard 2×2 fiber coupler (28)which are available from Gould and AMP. A 2×2 fiber coupler acts both asa standard Y coupler (29) and as a beam splitter (31). Light beamsentering the fiber coupler through its two input fibers are combined bya standard optical coupler to form a combined beam. The combined beamthen propagates along fiber within the fiber coupler to the beamsplitter of the fiber coupler. The beam splitter divides the combinedbeam into two beams. These two beams exit the fiber coupler through itstwo output fibers.

[0043] In the diagram of the depolarizer (24) shown in FIG. 3, lightfrom a light source, not shown, is directed into the input fiber (26) ofthe 2×2 fiber coupler (28). The light source can be any coherent lightsource, such as a laser diode. One of the two beams split from the inputbeam exits the fiber coupler (28) through the output fiber (30). Thisexiting beam will hereafter be referred to as the output beam. The othersplit beam exits the fiber coupler (28) though the fiber (34) whichforms part of the recirculation loop (32). The recirculation loop (32)may be formed by coupling one of the output fibers of the fiber couplerto one of the input fibers.

[0044] Thus, light entering the depolarizer (24) is sent along the inputfiber (26) to the fiber coupler (28) where the input beam is split bythe beam splitter into two beams. One of the split beams is sent alongthe output fiber (30) and the other split beam is sent along therecirculation loop (32). The beam sent on the recirculation loop (32),which will be referred to as the recirculation beam, is then recombinedwith the input beam within the fiber coupler (28). This combined beam oflight formed from combining the input beam and the recirculation beam isthen split by the splitter within the fiber coupler into two beams, asbefore, one of which is sent along the output fiber (30) and one ofwhich is sent along the recirculation loop (32). Thus, light from theinput beam is divided, recirculated, combined with the input beam,divided, recirculated, recombined with the input beam, and so on.

[0045] When considering the effect on the SOP of the output beam (30)due to the recirculation and recombination of the input beam (26), thefollowing occurs. Light passing through a fiber experiencesbirefringence, i.e. the SOP changes as the light propagates along thefiber. Thus, the initial SOP is altered each time light passes through afiber. In the depolarizer shown in FIG. 3 the initial SOP of the inputbeam (26) is altered as the beam passes through the fiber coupler (28)and as the beam passes along both the recirculation loop and the fiberof the output beam (30). While the initial output beam (the output beambefore recirculation beams are combined with the input beam) will have adifferent SOP than the input beam, the degree of polarization (DOP) isthe same. Each of the input and output beams has equal DOP but adifferent orientation, or SOP. As the recirculation beam (32) isrecombined with the input beam (26), the SOP of the light of therecirculation beam is different than the SOP of the input beam due tothe birefringence inherent in the fiber coupler (28) and therecirculation loop (32). Even as the birefringent effect changes withchanging environmental factors, the SOP of the recirculation beam willcontinue to be altered from the SOP of the input beam.

[0046] Thus, the combined beam, formed from coupling the input beam andthe recirculation beam within the fiber coupler, will no longer bepolarized in one particular direction, but will be the combination oftwo beams with different polarization states. As explained above, thiscombined beam is split into two beams by the beam splitter within thefiber coupler (28). This new output beam, being the combination of theinput beam and the recirculation beam, will then have not only adifferent SOP than the input beam but, as the combination of beams ofdifferent SOP, will also have a different DOP since this new output beamwill no longer be polarized in one SOP. Similarly, the recirculationbeam resulting from the splitting of the combined input beam andrecirculation beam will also not have the high DOP of the initial inputbeam.

[0047] This process is repeated continually as the beam within thedepolarizer is continually split, sent along the recirculation loop,recombined with the input beam, and input into the beam splitter. Thenet result of each split, recirculation and recombination is that theoutput beam is the combination of many beams of differing SOP and DOP,thereby reducing the degree of polarization of the output beam with eachrecombination.

[0048] If the recirculation loop is longer than the coherence length ofthe light of the input beam, then the input beam will not be coherentwith the recirculation beam. The coherence length of a light source is awell known quantity and is proportional to the inverse of the spectrallength. When two beams that are not coherent to each other are combined,they do not substantially interfere with each other. For a standardlaser diode with a wavelength of 1300 nm and a spectral length of 0.1nm, the coherence length is approximately 1.67 cm. Thus, as long as therecirculation loop is longer than 1.67 cm, the interferometric effectsof combining the input beam and the recirculating beam will benegligible. As n, the number of recirculations, approaches infinity,i.e. when the depolarizer has been connected to the light source formore than a few nanoseconds, the intensity of the light approaches theintensity of the input beam, assuming no internal or splicing losses.

[0049]FIG. 4 is a diagram illustrating three single-ring recirculatingdepolarizers of FIG. 3 connected in series such that the output from afirst depolarizer (36) is the input of a second depolarizer (38) and theoutput of the second depolarizer (38) is the input of a thirddepolarizer (40). The intensity of the output beam (42) is equal to theintensity of the input beam, assuming no internal losses or splicinglosses. Thus, even with three single-ring depolarizers connected inseries, the output beam has the same intensity as the input beam.

[0050] The embodiments shown in FIGS. 1 and 2, including the fibergrating embodiment of the depolarizer of FIG. 2, may also be connectedin series as shown in FIG. 4.

[0051]FIG. 5 shows a tunable single-ring depolarizer (44), where apolarization controller (46) is included in the input fiber (48). Thepolarization controller (46) is adjustable to control the polarizationof the input beam within the input fiber (48). Light from the inputfiber (48) that passes through the polarization controller (46) is inputto the 2×2 fiber coupler (50). The fiber coupler (50) splits the inputbeam from the input fiber (48) into two beams, which exit the fibercoupler (50) through fibers (52) and (54). Fiber (52) carries the outputbeam and fiber (54) connects to a polarization controller (56).Polarization controller (56) is also connected to the input fiber (58)of the fiber coupler (50). In this manner the polarization controller(56) and fibers (54) and (58) form the recirculation loop (60) of thetunable single-ring depolarizer (44). After passing through therecirculation loop (60), light exiting the fiber coupler (50) throughfiber (54) is recombined with the input beam from fiber (48) by thefiber coupler (50).

[0052] By adjusting both the polarization of the input beam by thepolarization controller (46) and the polarization of the recirculationbeam by the polarization controller (56), the DOP of the output beam canbe tuned to significantly lower DOP than was realizable with thesingle-ring depolarizer (24) shown in FIG. 3. The tunable single-ringdepolarizer (44) of FIG. 5 has been found to depolarize light to as lowas 1.15%. Significantly, the tunable single-ring depolarizer (44) may beadjusted to provide an output beam with a 99.8% polarization. Thus, thetunable single-ring depolarizer is able not only to provide a highlydepolarized light, but can also be tuned such that it provides nearlyperfectly polarized light. This gives the tunable single-ringrecirculation depolarizer great flexibility in applications were the DOPof the light would need to be varied or specifically tuned to aparticular DOP.

[0053]FIG. 6 illustrates three tunable single-ring depolarizers of FIG.5 connected in series. The output of one tunable single-ring depolarizeris connected to the input of the next tunable single-ring depolarizer.Each tunable single-ring depolarizer (64), (66) and (68) is constructedin the same manner as the tunable single-ring depolarizer (44) shown inFIG. 5, which includes polarization controllers (46) and (56) insertedin the input fiber (48) and recirculation loop (60) of the depolarizer(44). By utilizing three tunable single-ring depolarizers connected inseries, the degree of polarization of the light output from the thirdtunable single-ring depolarizer has been reduced as low as −20 dB.

[0054] Several possible types of polarization controllers could beutilized with the depolarizer (44). One possible depolarizer isdescribed in U.S. Pat. No. 4,389,090 issued to H. C. LeFevre, which isincorporated by reference herein. This type of polarization controllerutilizes two metal plates to twist two coils of fiber, thereby inducingbirefringence to change, and thereby control, the polarization of lightoutput from the polarization controller. Other possible polarizationcontrollers include liquid crystal polarization controllers andintegrated optics polarization controllers.

[0055] While the embodiments of multiple single-ring depolarizers shownin FIG. 4 and FIG. 6 are constructed from either the single-ringdepolarizer (24) shown in FIG. 3 or the tunable single-ring depolarizer(44) shown in FIG. 5, other embodiments of the present invention couldcombine depolarizers of the type (24) shown in FIG. 3 with tunabledepolarizers (44) of the type shown in FIG. 5. Additionally, otherembodiments of the present invention could combine more or fewersingle-ring depolarizers of either type, or both types, in any order. Asone possible example of this, the depolarizer (70) shown in FIG. 7 isconstructed from three single-ring depolarizers (72), (74) and (76) ofthe type shown in FIG. 3 and two tunable single-ring depolarizers (78)and (80) of the type shown in FIG. 5. In the embodiment shown in FIG. 7,the tunable single-ring depolarizers (78) and (80) occupy the third andfifth positions in the order of depolarizers along the path ofpropagation of the light beam through the fiber from left to right.

[0056] While the above multiple single-ring depolarizers connectedsingle-ring depolarizers in series, other embodiments of the presentinvention could connect recirculating depolarizers in differentconfigurations.

[0057] Specifically, FIG. 8 is a diagram illustrating a multiplesplitter recombination depolarizer (82) constructed from four 2×2 fibercouplers (84), (86), (88) and (90). Light is input to the fiber coupler(84) through input fiber (92). The fiber coupler (84) splits the beamfrom the input fiber into two beams. The two beams exiting the fibercoupler (84) are connected to the fibers (94) and (96). Fiber (94) isinput fiber coupler (90) and fiber (96) is input fiber coupler (86).Fiber coupler (86) recombines beams of light from fibers (108) and (96)and then splits the recombined beam into two beams. These two beams exitfiber coupler (86) through exit fibers (98) and (100). Fiber (98)connects to the input of fiber coupler (88). Fiber (100) serves as theoutput fiber of the depolarizer (82). The output beam (98) from fibercoupler (86) is combined by the fiber coupler (88) with the beamconnected from the fiber (102). Fiber coupler (88) splits the combinedbeam from fibers (98) and (102) into two beams. These two beams exitfiber coupler (88) through fibers (104) and (106). Fiber (104) is inputfiber coupler (84), fiber (106) is input fiber coupler (90). Fibercoupler (90) combines the beams of light from fibers (94) and (106) andthen splits the combined beam into two beams. These two beams exit fibercoupler (90) through fibers (102) and (108). Fiber (108) connects tofiber coupler (86) and fiber (102) connects to fiber coupler (88). Fibercoupler (84) combines the beam from fiber (104) with the beam from inputfiber (92). Fiber coupler (84) then splits the combined beam into twobeams which exit through fibers (94) and (96).

[0058] Polarized light entering the depolarizer (82) through input fiber(92) is repeatedly split and recombined with recirculated light beams asthe split and recombined beams circulate through the four fiber couplers(84), (86), (88) and (90). In this manner the output beam (100), whichis a combination of recirculated beams, is then the weighted average ofthe polarization states of all of the combined beams. Due to thebirefringence of light traveling along the fibers between the fibercouplers, the result of the weighted averaging of the polarizationstates of the combined beams produces an output beam with a very lowDOP.

[0059] While the embodiment shown in FIG. 8 utilizes four fibercouplers, other embodiments could utilize more or fewer fiber couplersarranged such that the light beam traveling through the depolarizer isrepeatedly split, sent along different optical paths with differentbirefringent effects, and recombined to average the differentpolarization states, thereby depolarizing the output light beam.Additionally, other embodiments of the present invention may usepolarization controllers or single-ring depolarizers with multiple fibercouplers connected in a non-series manner. The embodiment illustrated inFIG. 8 is provided to demonstrate the versatility of the presentinvention. As such, it is but one of many possible combinations inaccordance with the present invention.

[0060] While the above embodiments shown in FIGS. 2-8 utilized astandard narrow band laser diode, the present invention can also utilizebroad band light sources depending on the application. This flexibilityof the present invention, to use either broad band light sources ornarrow band light sources, with their lower cost and greaterreliability, gives the present invention an advantage over currentlyavailable optical depolarizers.

[0061] While the above embodiments shown in FIGS. 2-8 utilize standard2×2 fiber couplers due to their low cost and low internal loss, otherembodiments of the present invention may utilize separate beam splittersand couplers to split and recombine the light beam. Other embodimentsmay utilize 3×3 or 4×4 or greater fiber couplers to allow larger numbersof recirculation loops. While the fiber couplers of FIGS. 3-4incorporated a 50/50 beam splitter, this is used as an example only andother embodiments could utilize splitters with different splittingratios. While the recirculation loops of the embodiments shown in FIGS.3-4 are formed by splicing two fibers together, in other embodiments therecirculation loop could be formed from a single fiber.

[0062]FIG. 9 shows a single ring depolarizer (120) formed from a singlepiece of fiber (110) looped to form a recirculating loop (112). Lightinput the fiber (110), forming the input beam, propagates along thefiber and through the recirculation loop (112) to the coupling point(114) where the fiber (110) is coupled with itself. Light propagatingalong the recirculation loop (112) is split into two beams at thecoupling point (114). The beam propagating along the recirculation loop(112) is referred to as the recirculation beam. One beam propagatesalong the output end (116) of the fiber to form the output beam of thedepolarizer (120). The other beam propagates along the recirculationloop (112) of the fiber (110). The birefringence of the recirculatingloop alters the SOP of recirculation beam from the SOP of the inputbeam. Combing the recirculation beam with the input beam at the couplingpoint (114) results in a lowering of the DOP of the output beam of thedepolarizer (120).

[0063] The coupling of the fiber (110) with itself can be performed bymerging the fibers together, typically after some of the fiber'scladding has been removed, to allow light to pass between the lightcarrying cores. Processes including heating, polishing or chemicalmerging may be employed to this end.

[0064] While the embodiments disclosed herein used single mode fiber,the present invention is not limited to single mode fiber and otherembodiments could be constructed from other types of optical fiber.Additionally, several different types of optical fiber can be used inone depolarizer.

[0065] Additionally, while the present invention utilizes single modefibers due to their low cost and availability, other embodiments of thepresent invention may utilize other types of fiber or other media torecirculate and transmit the beam path.

[0066]FIG. 10 shows a depolarizer (122) formed as part of an integratedoptical chip (124). Several materials are available for waveguideformation, such as LiNbO₃ or plastics. Light from an input source, notshown, is directed into the input waveguide (126) and forms the inputbeam. The input beam propagates along the waveguide (126) to the point(128). At the point (128) the waveguide splits into a output waveguide(130) and a recirculation waveguide (132). The input beam is split intotwo beams when it reaches the point (128). One of the split beamspropagates along the output waveguide (130) and forms the output beam ofthe depolarizer (122). The other split beam propagates along therecirculation waveguide (132) and forms the recirculation beam. Apolarization controller (136) is included in the recirculating waveguideand alters the SOP of the recirculation beam. The recirculation beam iscombined with the input beam at the point (134). The combinedrecirculating beam and input beam propagates to the point (128) where itis split into two beams, as described above. The output beam propagatingalong the output waveguide (130) is then formed from the combination ofthe input beam and the recirculation beam.

[0067] In this way a portion of the input beam is diverted along arecirculation path which, due to the polarization controller (136),alters the SOP of the light. When the recirculation beam is combinedwith the input beam, the DOP of the combined beam is lower than the DOPof the input beam. Each time the combined beam reaches the point (128),a portion of the combined beam is diverted along the recirculationwaveguide (132). Each time the beam from the recirculation waveguide(132) is combined with the input beam, the DOP of the resulting combinedbeam is lowered due to the change in the SOP of the beam propagatingalong the recirculating waveguide. The resulting output beam of thedepolarizer, as the combination of beams with different SOP, has a lowerDOP than the input beam.

[0068] The polarization controller (136) can be of either an electricfield or acoustic type. In the electric field or acoustic typepolarization controller either an electromagnetic wave or an acousticwave is used to change the index of refraction of a medium within thepolarization controller, thereby altering the SOP of a beam of lightpassing thought the polarization controller.

[0069] While the embodiment shown in FIG. 10 utilizes a waveguidesformed on a substrate, other embodiments could have waveguides which areformed in a substrate by etching and deposition methods or otherprocesses. Practitioners of the art of integrated optics will appreciatethe many materials and methods used in forming integrated opticaldevices which are applicable in forming an integrated opticaldepolarizer in accordance with the present invention. As such, theembodiment shown in FIG. 10 is given as a representative example and isnot meant to be limiting on the materials or methods used in forming anintegrated optical depolarizer.

[0070] While the embodiment shown in FIG. 10 has only one recirculationloop, other embodiments could have multiple recirculation loops andmultiple polarization controllers. In an embodiment with multiplerecirculation loops, such loops may be positioned on both sides on theinput and output waveguides, as one loop is formed on one side in FIG.10. In another possible arrangement, multiple loops could be positionedoil the same side of the input and output waveguides. Additionally, thedepolarizer (122) of FIG. 10 can be connected in series with otherdepolarizers formed on the same substrate or on separate substrates.

[0071] While the above embodiments have assumed negligent splicinglosses and internal losses for explanation purposes, as with all opticsystems and devices, the actual losses of a depolarizer in accordancewith the present invention will vary depending on the materials selectedas well as the methods and quality of construction.

[0072] Although the invention has been described in conjunction withparticular embodiments, it will be appreciated that variousmodifications and alterations may be made by those skilled in the artwithout departing from the spirit and scope of the invention. Inparticular, those skilled in the art will recognize that the presentinvention is not limited fiber optic communications and opticalinstruments.

What is claimed is:
 1. An optical depolarizer comprising: an input forreceiving an input light; a splitter; a recirculation loop having a loopoutput, wherein the input light is split by the splitter into at leasttwo beams and wherein one of the at least two beams is directed alongthe recirculation loop; and means for combining the loop output with theinput light.
 2. The depolarizer of claim 1 , wherein a combined outputof the means for combining is input to the splitter.
 3. The depolarizerof claim 1 , wherein a first splitter output is input to a loop input ofthe recirculation loop.
 4. The depolarizer of claim 2 , wherein a firstsplitter output is input to a loop input of the recirculation loop. 5.The depolarizer of claim 1 , further comprising: a polarizationcontroller coupled to the input of the depolarizer.
 6. The depolarizerof claim 1 , further comprising: a polarization controller insertedwithin the recirculation loop.
 7. An optical depolarizer, comprising: aN×N fiber coupler having N inputs and N outputs; and a recirculationloop formed from coupling at least one output with at least one input.8. The depolarizer of claim 7 , further comprising: a first polarizationcontroller coupled to the input of the depolarizer; and a secondpolarization controller inserted within the recirculation loop.
 9. Anoptical depolarizer, comprising: a plurality of N×N fiber couplers, eachfiber coupler having N inputs and N outputs; and at least onerecirculation loop formed by coupling at least one output to at leastone input.
 10. The depolarizer of claim 9 , wherein the at least onerecirculation loop couples at least one output to at least one input ofthe same fiber coupler.
 11. The depolarizer of claim 9 , furthercomprising: at least one polarization controller coupled to an input ofthe at least one fiber coupler; and at least one polarization controllercoupled to the at least one recirculation loop.
 12. The depolarizer ofclaim 9 , wherein N=2.
 13. A method for depolarizing a light source,comprising the steps of: splitting an input light beam into a pluralityof light beams; recirculating at least one of the plurality of lightbeams along a birefringent path; and combining at least one of therecirculated light beams with the input light beam.
 14. The method ofclaim 13 , wherein the recirculated light beam is combined before theinput light source is split into a plurality of light beams.
 15. Themethod of claim 13 , wherein the polarization of the input light beam iscontrolled by a polarization controller.
 16. The method of claim 13 ,wherein the polarization of the recirculated light beam is controlled bya polarization controller.
 17. an optical depolarizer, comprising: anoptical fiber having an input end and an output end, wherein the opticalfiber is coupled with itself to form a recirculation loop between theinput end and the output end.
 18. An optical depolarizer formed as anintegrated optical device on a substrate, comprising: an input; anoutput; a recirculation loop, wherein a portion of light entering theinput is directed into the recirculation loop; a polarization controllerpositioned along the recirculation loop for altering the polarization oflight exiting the recirculating loop; and a means for combining lightexiting the recirculation loop with light entering the input, wherein atleast a portion of the combined light is directed into the output. 19.An optical depolarizer of claim 18 , wherein a portion of the combinedlight from the input and the recirculation loop is propagated along therecirculation loop.
 20. An optical depolarizer, comprising: abirefringent element; a two-way mirror; and a partially reflectingmirror, wherein the birefringent element, two-way mirror and partiallyreflecting mirror form a recirculation segment.
 21. An opticaldepolarizer formed from a segment of optical fiber, comprising: a firstgrating section; a second grating section; and a recirculating segment.22. An optical depolarizer of claim 21 , wherein a portion of the lightpropagating along the fiber is transmitted through the first gratingsection to the second grating section, wherein a portion of the lightstriking the second grated section is reflected back to the first gratedsection and a portion of the light striking the second grated section istransmitted through the second grated section, wherein a portion of thelight striking the first grated section after being reflected back tothe first grated section from the second grated section is reflectedback toward the second grated section and thereby combined with lighttransmitted through the first grated section, and wherein the portion ofthe light transmitting through the second grated section forms an outputbeam of the depolarizer.